Dioxygen uptake by Co(salen)

Naturally occuring dioxygen carriers and storage proteins contain a transition metal ion to which dioxygen can reversibly bind, eg. Iron (myoglobin,Mb, haemoglobin, Hb) or Copper (haemocyanin). In this experiment a simple cobalt complex will be prepared which also reversibly binds dioxygen. Many complexes of this type have been used as "models" to aid in the understanding of how the proteins function.
When Co(salen) was first prepared in 1933, it was observed that the red-brown crystals darkened on exposure to air. However, it was not until five years later that it was established that the colour change was due to reversible uptake of dioxygen. H2salen (I) is a Schiff-base ligand formed by the condensation of two molecules of salicylaldehyde with 1,2-diaminoethane (ethylenediamine).

In 1944, it was found that different crystalline forms existed depending on the solvent used in the preparation or for recystallisation and that these had varying capacity for oxygenation in the solid state. This variation in oxygenation has been related to the presence of voids in the crystal lattice, sufficient to allow the passage of molecular oxygen. This suggestion is supported by the X-ray crystal structure determination of the so-called "inactive" form which shows that the structure consists of di meric units [Cosalen]2, (II).
inactive form(II)

The active forms of Cosalen are presumed to contain dimeric units with open lattices packing relative to the inactive form. One form (III) has one Co atom directly above the other.

active form(III)

The importance of solid-state packing effects in determining oxygenation ability is further indicated by kinetic studies of dioxygen uptake at the crystal surfaces of the different forms of Co(salen). Measurements involving different temperatures and pressure conditions have shown that, following an induction period and after attainment of equilibrium, the kinetics of dioxygen absorption at the surface were no longer important and the uptake was controlled only by the rate of diffusion of dioxygen into the crystal.
In solution, it has been found that, depending on the solvent, in the absence of dioxygen, the cobalt(II) may be four, five or six coordinate. For example, in a strongly coordinating solvent such as pyridine, both [Co(salen).pyr] and [Co(salen).2pyr] exist, whilst in chloroform, the major species appears to be Co(salen). Irrespective of the solvent, the rate of dioxygen uptake appears to be similar, however the product obtained may be a 1:1 (IV) or a 2:1 (V)(oxygen bridged) complex.

dioxygen complex(IV) bridged dioxygen complex(V)

In this experiment, the inactive form of Co(salen) is prepared. The uptake of dioxygen is then investigated for the complex in DMSO solution to establish whether a 1:1 or a 2:1 complex is formed under these conditions.



To a solution of salicylaldehyde (2.1 cm3) in 25 cm3 boiling ethanol is added 1,2-diaminoethane (0.7 cm3). The reaction mixture is thoroughly stirred for 3-4 minutes and the solution then left to cool in an ice-bath. The bright yellow flaky crystals are filtered under suction and washed with a small volume of ice-cold ethanol, then air-dried. The yield and the melting-point should be recorded.


This preparation is sensitive to air, so should either be performed whilst flushing a stream of nitrogen gas through the flask or under vacuum. The arrangement for working under reduced pressure is given below.
Dissolve H2salen (1.6 g) in 60 cm3 of ethanol at 60-70°ree;C in a side-arm flask fitted with a clamp. The side-arm is connected to an aspirator. Upon dissolution of the ligand, quickly add Cobalt(II) acetate tetrahydrate (1.25 g) in 7 cm3 of ethanol:water mixture, while swirling the flask. Immediately stopper the flask and evacuate through the side-arm for a short while. NOTE: The flask MUST be securely clamped since the ethanol mixture has a tendency to bump vigorously. Be careful not to draw off the bulk of the ethanol and clamp the side-arm once reduced pressure has been established. Continue heating with periodic swirling for 1-2 hours.
During this period, the initially formed brown "active" complex slowly changes to the brick-red "inactive" complex. Once this has occurred, cool the solution to room temperature, collect the crystals on a sintered glass filter funnel (in air) and wash three times with 7 cm3 of ice-cold ethanol. Dry in a dessicator, then record the yield.
An IR spectrum from a typical student preparation is available.

Dioxygen Absorption by Co(salen) in DMSO


Accurately weigh out a sample of Co(salen) which has been finely ground (between 0.05 and 0.1 g) and place it in a side-arm test tube. Transfer DMSO (approximately 5 cm3) to a small beaker and bubble oxygen through it for a few seconds. (CAUTION: although DMSO is not itself poisonous, it is readily absorbed by the skin and can easily carry other compounds through the skin with it). Now transfer this DMSO to a small test tube that can fit inside the side-arm test tube and lower the tube carefully inside without spillage.
Connect up the apparatus as shown in the diagram, so that the movable arm reservoir can be adjusted to bring the water level of the graduated tube near the bottom.
experimental setup

Flush the side-arm tube with a gentle stream of oxygen. Insert a tightly fitting rubber stopper in the mouth of the tube. Adjust the movable arm to make the water levels equal in both sides (ie ensuring that the pressure within the apparatus is atmospheric). Record the water level in the graduated tube.
Finally, carefully invert the side-arm tube (holding near the stopper to minimise heating by the hand) and record the time. The DMSO should be allowed to dissolve the Co(salen) but not be spilled into the side-arm. As oxygen is absorbed, the water level in the graduated tube begins to rise. Note the changes occurring in the tube. Continue shaking until no further change in water level occurs (taking a reading every two minutes for 20 minutes is usually sufficient). Adjust the moveable arm before each reading so that the water levels in the tubes are again equal.
Draw a graph of volume changes versus time and extrapolate to estimate the overall total oxygen uptake. From this volume change at room temperature and atmospheric pressure, the number of mole of dioxygen absorbed per mole of Co(salen) can be calculated.

Electron Paramagnetic Resonance

Suppose that an electronic spin system of S=0.5 is brought under the influence of a static magnetic field H. The energy state of the system would split into two, Ms=+0.5 and Ms=-0.5, owing to the interaction of the spin system with the magnetic field. The energy gap between the two states is ΔE = 2βH, where β is in Bohr Magnetons, the unit of electron spin moment. The majority of the spins of the system are in the lower energy state. When an electromagnetic wave of frequency v is applied to this system and a condition ΔE=hν is satisfied, an excitation from the lower to the upper level occurs. As a result, a fraction of the energy of the electromagnetic wave will be absorbed by this system. For a free unpaired spin then,
    ΔH= hv=2βH
Usually a microwave of 3.2 cm wavelength (called the X-band) is used for EPR. Then v ~ 9200 MHz. By putting explicit values for Β and h, we find that H =3200 gauss. The resonance condition is more generally expressed using the g value ie,
    ΔH= hv=gβH
where g is called the spectroscopic splitting factor and is equal to 2.00229 for a free radical. Values of g for a formal orbitally singlet ground state can be expressed by;
where λ is the effective spin-orbit coupling constant, Δ is the energy gap between the ground level and the excited level in question, and n is a constant depending on the two levels concerned. Thus, the g value may afford information about the effective value of λ and the energy gap Δ. The former has been interpreted to give a measure of covalency of the bond between the central metal ion and the ligands.
The g value can vary from one direction to another ie, if x-, y- and z-axis are defined in a complex, gx, gy and gz may not be the same. In a strictly octahedral complex they should be equal (isotropic), but if the symmetry is lowered to tetragonal then gx=gy =gz. For less than axial symmetry then gx=gy=gz. This means that the fundamental symmetry of the ligand field may be deduced from the shape of the EPR spectrum.
Co(II) has a d7 configuration and for low spin complexes S=0.5. l is negative and generally gx>gy>gz. To complicate the appearance, the hyperfine interaction with the nucleus of the Co atom, which has I=7/2, splits each band into eight peaks (2I+1). In most cases, Ax and Ay are small and the hyperfine structures on gx and gy are not well defined, while the hyperfine on gz gives well separated peaks.
A rough estimate of g and A can be obtained from the spectrum, but for precise values computer simulations need to be performed.
The curves shown below are from computer simulation exercises and also display "stick spectra" indicating the positions for the measurement of the A values.
1) Co(salen) in pyridine
deoxygenated EPR spectrumdeoxygenated form of Cosalen

Note that the best resolved peaks show superhyperfine splitting due to the presence of a N from one coordinated pyridine group (IN=1). The g values obtained were 2.464, 2.244 and 2.024.

2) A simulation of the dioxygen adduct of a Co(II) substituted Salen complex.

oxygenated EPR spectrumoxygenated form of Cosalen

Note the expanded scale. The g values obtained here were 1.994, 2.010 and 2.081.


1. Sketch the spectra and carefully label the positions of the g and A values. Calculate the A values from the separations of the stick spectra.
2. The relative sizes of the A values has been used to predict the occupancy of the unpaired electron on the Cobalt in the dioxygen complex. Explain
3. Similar EPR spectra have been obtained from cobalglobin (obtained by replacing Fe by Co in Haemoglobin). On this basis, it has been suggested that the Cobalt model compounds are electronically good models for Hb. What other factors need to be considered before these complexes can be considered good models for all aspects of Hb.



F.A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", Fifth Edition,Wiley-Interscience, New York, 1988, pages 735-738.
T Appleton, J. Chem. Educ., 54, 1977, 443.

X-Ray Structures

N.G. Vannerburg, Acta Cryst., 18, 1965, 449.
S. Bruckner, M. Calligaris, G. Nardin and L. Randaccio, Acta Cryst., B2, 1969, 1971.
R. Delasi, S.L.Holt and B. Post, Inorg. Chem., 10, 1971, 1498.
W.P. Schaefer and R. E. March, Acta Cryst., B25, 1969, 1675.
M. Calligaris, G. Nardin, L. Randaccio and A. Ripamonti, J. Chem, Soc., A (1970), 1069.

Electron Paramagnetic Resonance

T.D. Smith and J.R. Pilbrow, Coord. Chem. Rev., 39, 1981, 295.
R.J. Lancashire, T.D. Smith and J.R. Pilbrow, J. Chem Soc. Dalton Trans., 1979, 66.

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